1. Field of the Invention
The present invention relates to a single-headed piston type refrigerant compressor of the type in which rotation of a swash plate is converted into a reciprocation of a plurality of single-headed pistons via a plurality of pairs of shoes arranged between an outer periphery of the swash plate and the single-headed pistons. The present invention also relates to a method of producing a swash plate suitable for being incorporated in the above-mentioned type of single-headed piston type refrigerant compressor.
2. Description of the Prior Art
A swash-plate-operated refrigerant compressor, either a double-headed piston type refrigerant compressor or a single-headed piston type refrigerant compressor, has a housing assembly including a cylinder block provided with a plurality of cylinder bores formed therein, a plurality of pistons respectively slidably fitted in the cylinder bores, a drive shaft supported by the housing assembly to be rotatable about an axis of rotation, and a swash plate fixedly mounted on the drive shaft within a crank chamber at a constant inclination with respect to a plane perpendicular to the axis of rotation of the drive shaft or mounted on the drive shaft so that its inclination can be adjustably changed in the crank chamber. A part of each piston, i.e., a substantially middle part if the piston is of a double-headed type or an end part opposite the compressing end surface if the piston is of a single-headed type, is connected to a peripheral part of the swash plate via a pair of shoes to provide an operative engagement between each piston and the swash plate. This operative engagement of each piston and the awash plate permits the conversion of rotating motion of the drive shaft and the swash plate into a reciprocating motion of each piston.
In this regard, it is an important technical problem to avoid seizure between the front and the rear surface of the swash plate and the pair of shoes as well as to reduce friction between contacting portions of the swash plate and the shoes to the least possible extent. A refrigerant gas entraining a lubricating oil mist is circulated through the swash plate compressor to lubricate movable components of the compressor. However, in an initial stage of operation of the compressor at a low temperature, the refrigerant gas washes off the lubricating oil remaining on the sliding surfaces of the swash plate before the lubricating oil mist reaches the swash plate and hence the surfaces of the swash plate are in a dried-surface condition having no lubricating oil, and therefore the swash plate and the shoes must unavoidably start to slide relative to each other without lubrication. Thus, the swash plate must be exposed to a very severe operating condition during the initial stage of sliding motion thereof. Moreover, a new refrigerant, such as R134a, which has recently become used instead of the conventional refrigerant for the protection of the ozonosphere is more effective in creating a dried-surface condition than the conventional refrigerant. Accordingly, demand for an improvement in the lubricating property of the surfaces of the swash plate has progressively increased.
Conventional methods intended to satisfy the above-mentioned demand by applying a surface treatment process to a swash plate have been proposed in Japanese Unexamined Patent Publication (Kokai) No. 60-22080 (Japanese Examined Patent Publication No. 5-10513), International Publication WO95/25224 and Japanese Unexamined Patent Publication (Kokai) No. 8-199327.
The typical conventional method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-199327 includes forming of a sprayed metal coating of a copper-base or aluminum-base material on a swash plate made of a base metal, and forming of a plated coating of lead-base material or a film of a polytetrafluoroethylene over the sprayed metal coating. The plated film or the film of the polytetrafluoroethylene is formed over the surface of the sprayed metal coating in order to improve the antiseizing property of the sprayed metal coating and to prevent the sprayed metal coating from cracking.
Although the foregoing cited references disclose diverse techniques for the surface treatment of a swash plate, nothing is suggested in these techniques about means for securing compatibility between the surface treatment and management of thickness of the swash plate. For example, the afore-mentioned Japanese Unexamined Patent Publication (Kokai) No. 8-199327 discloses the technique of plating or coating the uppermost surface of a swash plate, but teaches nothing about the management of the thickness of the plated layer or the film in relation to an accurate management of the entire thickness of the swash plate.
Generally, in the swash-plate-operated refrigerant compressor, a special consideration is provided to determination of the amount of stroke of the pistons, in order to reduce a top clearance between each piston and a valve plate assembly when the piston is at its top dead center, i.e., the minimum volume of the cylinder bore during the compression stroke of the piston, to the smallest possible amount near zero. In view of the piston driving principle of the swash plate compressor, the determination of the amount of stroke of each piston largely depends on a production accuracy in the thickness of the swash plate with respect to a designed thickness. Therefore, if an appropriate surface treatment method is applied to the surface of the swash plate to reduce sliding friction between the surfaces of the awash plate and the shoes, a final object of improving the compression efficiency of the swash plate type compressor cannot be achieved if the surface treatment method makes it difficult to control the thickness of the swash plate.